This invention relates to a process for the synthesis of alkylene carbonates. More specifically the invention relates to a process for the synthesis of alkylene carbonates by reacting alkylene oxides with carbon dioxide in the presence of a porous solid support catalyst containing an alkali or alkaline earth metal component.
Alkylene carbonates, such as ethylene carbonate and propylene carbonate, are useful as specialty solvents, hydraulic fluid additives and intermediates in numerous chemical processes. For example, ethylene carbonate is useful as an intermediate in polycarbonate manufacture, as discussed more fully below, as well as in the manufacture of ethylene glycol.
The traditional method for the production of polycarbonate resin employs phosgene and bisphenol-A as starting materials. However, this method has numerous drawbacks, including the production of corrosive by-products and safety concerns attributable to the use of the highly toxic phosgene. As such, polycarbonate manufacturers have developed non-phosgene methods for polycarbonate production, such as reacting bisphenol-A with diphenyl carbonate that can be synthesized from dimethyl carbonate and phenol.
Dimethyl carbonate has a low toxicity and can be used to replace toxic intermediates, such as phosgene and dimethyl sulphate, in many reactions, such as the preparation of urethanes and isocyanates, the quaternization of amines and the methylation of phenol or naphthols. Moreover, it is not corrosive and it will not produce environmentally damaging by-products. Dimethyl carbonate is also a valuable commercial product finding utility as an organic solvent, an additive for fuels, and in the production of other alkyl and aryl carbonates.
Dimethyl carbonate, as well as other dialkyl carbonates, have traditionally been produced by reacting alcohols with phosgene. These methods have the same problems as methods that use phosgene and bisphenol-A, i.e., the problems of handling phosgene and phosgene waste materials. Therefore, non-phosgene methods for the production of dimethyl carbonate, as well as other dialkyl carbonates have been developed. For example, U.S. Pat. No. 5,498,743 discloses a method for producing dialkyl carbonates, such as dimethyl carbonate, by reacting alkylene carbonates with alcohols. Thus, there is a significant market for commercially viable methods for the production of ethylene carbonate, as well as other alkylene carbonates, for use as intermediates in such a process.
Reaction of alkylene oxides with carbon dioxide in the presence of various catalysts to produce alkylene carbonates is known in the art. For example, homogeneous catalysts such as ammonium, phosphonium and sulphonium salts; a combination of protic compounds and nitrogen-containing bases; arsonium halides; tertiary phosphines; nitrogen bases; and alkali or alkaline earth metal halides have been proposed.
However, there are problems associated with using many of these proposed homogeneous catalysts in an industrial process. The problems include low selectivities to the desired alkylene carbonate, long reaction times (e.g. 5 hours or more) and the use of a relatively high amount of catalyst. Moreover, many of these catalysts can lose their activity during recycling and, therefore, require the disposal of a large amount of inactive spent catalyst.
Additionally, while some of these catalysts may provide adequate selectivities and reaction rates, they often are difficult to separate from the product stream or from by-products of the reaction. For example, a continuous process for the preparation of ethylene carbonate from ethylene oxide and carbon dioxide, using a potassium iodide catalyst, exhibits such a problem. Although the use of potassium iodide as a catalyst in such a process may provide an adequate selectivity and reaction rate, a significant amount of the potassium iodide is generally lost due to the difficulty of recovering the catalyst from the by-product or purge stream. Specifically, this reaction, between ethylene oxide and carbon dioxide, will produce polyglycols as by-products, which will generally be removed by way of a purge stream. Since the potassium iodide is highly soluble in these polyglycols, it is difficult to separate from them. As a result, the potassium iodide will either be removed with the polyglycols or require additional equipment and expense to separate the potassium iodide from the polyglycols.
The use of heterogeneous catalysts in the reaction of alkylene oxides with carbon dioxide has also been proposed. Examples of such heterogeneous catalysts include an anion exchange resin having a quaternary ammonium salt as an exchange group and a heteropolyacid based on an oxide of tungsten or an oxide of molybdenum and a salt thereof. Although such catalysts may avoid some of the problems associated with the homogeneous catalysts, as discussed above, they have relatively slow kinetics and can only be regenerated under limited conditions.
Thus, there is a need for an economical method of producing alkylene carbonates which does not have the above-mentioned disadvantages.